The invention is related generally to measuring systems and more particularly, to pressure measuring systems using optical fibers.
The measurement of blood pressure and other physiological pressures is an important technique in modern health care. The intravascular measurement of blood pressure results not only in the accurate measurement of the diastolic and systolic pressures produced in other techniques but also produces the entire pressure waveform. Many pressure measurement devices include a catheter having a tip containing a pressure transducer. Extending through the catheter body are conductors for conducting the pressure data signals from the transducer. At its proximal end, the catheter is coupled to equipment for processing the pressure data signals provided by the catheter tip into a visual display and into written records, if desired, for immediate or future use. In some catheters, the conductors comprise light guides which are referred to herein as optical fibers.
Optical fiber systems offer several advantages. For example, optical fibers are of relatively small size, they are immune to electromagnetic interference, and their cost is relatively low. These advantages make them attractive for use in medical applications, especially where disposability is desired.
In some prior pressure sensor systems, the catheter includes a diaphragm in its tip which moves in some manner in response to the patient's pressure. The diaphragm itself may have a light reflective surface or may be coupled to a light reflective surface which moves in response to diaphragm movement. Typically, in a two fiber system, two optical fibers traverse the catheter, one of which is a source fiber which emits light for reflection by the diaphragm-related reflector and another of which is a signal fiber which receives the reflected light. By measuring the amount of reflected light recieved by the signal fiber, the position of the diaphragm can be determined and, from this, the pressure can be derived.
When subjected to bending, many optical fibers will attenuate conducted light; that is, a significant amount of light is conducted out of the fiber near the bend, thus less light is conducted out of its end. This could cause less light to be emitted from the source fiber, less reflected light to reach the signal fiber and less reflected light to reach the processing equipment. Likewise, a light source which emits more or less light during periods of temperature change, electrical power changes, aging, or under other conditions may result in less reflected light from the reflective surface and less light received by the signal fiber. These changes may be interpreted as pressure changes and result in inaccurate measurements unless some type of compensation system is provided.
One prior compensation system involves the use of two additional fibers traversing the catheter which are coupled together at the transducer end of the catheter. One such fiber is the reference-source fiber and it is coupled at its proximal end to the same light source as the source fiber. The other such fiber is a reference-signal fiber which receives the light directly from the reference-source fiber at the transducer end; that is, the light is not first reflected by the diaphragm but is directly coupled from the reference-source fiber to the reference-signal fiber. These reference fibers would experience any bending and other conditions experienced by the actual signal and source fibers of the catheter and their signals may be used to compensate for such adverse conditions. While this system results in greatly improved accuracy, such an arrangement poses some manufacturing difficulties and increased expense. A catheter with four fibers is generally more expensive and difficult to manufacture than one with three fibers. Additionally, shorting the two reference fibers together in the vicinity of the transducer poses some manufacturing difficulties and expense. Such a catheter also is larger in size; i.e., diameter, to accommodate four fibers and because of the four fibers, is less flexible.
Another prior technique involves the use of only three fibers. Two fibers are used as the source and signal fibers as in the above technique. The third fiber is a reference-signal fiber which extends throughout the catheter as do the other two fibers and is used to compensate for any bending effects and other adverse conditions experienced by the other fibers. In this technique, a moveable diaphragm at the catheter tip is used to reflect light from the source fiber to the signal fiber and a stationary reflective surface also located at the catheter tip is used to reflect a portion of the source fiber's light to the reference-signal fiber. While accuracy is improved over the prior two-fiber approach, fabricating this second reflective surface in the catheter results in manufacturing difficulties and increased expense.
Hence those concerned with the development, manufacture and use of measurement systems have recognized the need for an improved measurement system using fewer optical fibers while still providing a means for compensating for light source output variations and other conditions which cause light variations which may result in inaccurate measurements. Additionally, those concerned have also recognized a need for a measurement system which is easier and less expensive to manufacture. The present invention fulfills these needs.